The present invention pertains generally to optical communication devices and pertains more particularly to optical waveguides that provide Raman amplification.
Current telecommunication and computer network services demand high-capacity communication systems. This demand is often satisfied by optical communication systems in which voice and data signals, represented by optical signals, are conveyed through optical waveguides such as silica fibers. Many optical communication systems are able to achieve better performance by using one or more types of optical amplifiers to increase the intensity of the optical signals. Typically, these optical amplifiers are driven by a source of optical energy known as pumping energy.
The demand for communication system bandwidth is growing rapidly. One technique known as wavelength division multiplexing (WDM) is often used in optical communication systems to meet this growing demand in a cost-effective manner by more fully utilizing the capacity of existing communication facilities. In many practical communication systems, however, the increase in capacity achieved by WDM is restricted by the fact many optical amplifiers can provide gain within only a fairly narrow bandwidth.
The erbium-doped fiber amplifier (EDFA) is one common type of optical amplifier that exhibits this limitation. An EDFA can provide reasonable gain for optical signals having a wavelength of about 1.5 xcexcm but it cannot provide a useful amount of gain for optical signals having a wavelength of about 1.3 xcexcm. As a result, the 1.3 xcexcm portion of the bandwidth is generally underutilized in optical communication systems that incorporate EDFA. This is particularly unfortunate in systems that use silica fiber because signal losses in such fiber are usually lower for wavelengths at 1.3 xcexcm than they are for wavelengths at 1.5 xcexcm.
This limitation in bandwidth can be avoided by using a so called Raman amplifier that achieves amplification through a phenomenon known as Raman scattering. Like EDFA, a Raman amplifier requires a source of pumping energy; however, the level of pumping energy must be considerably higher to achieve reasonable gain. Unlike EDFA, no special doping is required for a Raman amplifier. This feature is particularly attractive because a Raman amplifier can be incorporated into existing optical fibers by merely providing a suitable source of pumping energy. Furthermore, unlike the amplification provided by an EDFA, Raman amplification occurs over a fairly wide bandwidth that is to a large extent dependent upon only the wavelength of the pumping energy.
Unfortunately, a Raman amplifier can be noisier than many other types of optical amplifiers like an EDFA, for example. One source of noise is the Raman scattering mechanism itself, which readily couples intensity fluctuations of the pumping energy into the signal to be amplified. This problem can be mitigated by using counter-propagating pumping energy, which propagates in a direction counter to or opposite the propagation direction of the signal to be amplified.
Another source of noise in Raman amplification is due to variations in amplifier gain caused by fluctuations in the polarization orientation of the pumping energy. Although polarization-induced gain effects tend to be averaged along the length of a Raman amplifier, there are some situations in which the averaging effect does not occur because a particular polarization orientation exists throughout an appreciable portion of the amplifier length.
Although wide-bandwidth Raman amplification is possible in principle, a Raman amplifier is not very useful in a communication system if the spectral gain characteristic or spectral shape of the gain profile across frequency is highly irregular or nonuniform. As may be understood from the discussion above, the spectral gain characteristic of a Raman amplifier is determined essentially by the spectral shape and intensity of the pumping energy. Unfortunately, it is generally more costly to provide pumping energy having an appropriate polarization and sufficient intensity at the proper wavelengths that causes Raman amplification to have a reasonably flat spectral gain characteristic.
It is an object of the present invention to provide for a wide bandwidth optical amplifier having a reasonably flat spectral gain characteristic.
According to one aspect of the present invention, an optical amplifier comprises an optical waveguide having a first end and a second end, wherein a signal is received through the first end, is amplified by Raman amplification as it propagates within the optical waveguide from the first end to the second end, and is transmitted through the second end; a pumping energy source optically coupled to the optical waveguide to provide pumping energy that causes the Raman amplification to have a spectral gain characteristic; and a reflector having an input optically coupled to the second end of the optical waveguide and having an output, wherein the reflector receives the amplified signal through the input and reflects it through the output, and wherein the amplified signal is modified by the reflector according to a property that is complementary to the spectral gain characteristic of the Raman amplification.
The various features of the present invention and its preferred implementations may be better understood by referring to the following discussion and the accompanying drawings in which like reference numerals refer to like elements in the several figures. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention.